Grid energy storage (also called large-scale energy storage) refers to the methods used to store electricity within an electrical power grid. Electrical energy is stored during times when production (from power plants) exceeds consumption. The excess energy stored can be used at times when consumption exceeds production. Using such techniques, electricity production need not be drastically scaled up and down to meet consumption.
There are a variety of forms used for energy generation and storage. Some well-known devices include hydroelectric dams (using a water reservoir), combined cycle units (e.g., combined gas and steam turbines), gas turbines, coal units, pumped storage hydro (pumping water to a high elevation storage reservoir), flywheels (storing energy using a rotational disc), compressed air, batteries (e.g., sodium-sulfur, lithium-ion, flow, etc.), super capacitor bank, etc. Intermittent energy sources, such as wind turbines, photovoltaics, and tidal, can also provide energy to the grid, but such sources are by nature unpredictable, as production can vary not only seasonally and regionally, but from minute to minute.
Some such forms of energy storage and energy generation react slowly to changes in an input regulation signal, while others react quickly. For example, hydroelectric, coal, gas and combined cycle units are traditionally very slow in reacting to a change in regulation signals (1-6 minutes to exceed 90% capacity), while batteries and flywheels can react much faster (less than 1 minute, but can be 5 seconds or less to exceed 90% capacity), and, in some cases, nearly instantaneously.
Hybrid energy storage systems combine both fast and slow units. Based on a regulation input signal, a control algorithm determines the optimal distribution of the requested regulation on the participating units. The algorithm calculates operating points for each unit, which are then supplied to the unit models. For combined hydro and flywheel models, the set point is modified dynamically to compensate for hydro plant delays.
Nonetheless, the control algorithm still suffers in that the slow unit tracks the regulation signal substantially all of the time. Such tracking places a high burden and wear on the slow units.
Thus, a more efficient controller is desirable for hybrid energy storage systems.